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<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        //         找出数组中 元素为10的下标，有则打印该下标，没有则打印-1

        // * 例如: [88,20,10,100,50]  打印 2
        // * 例如: [88,20,30,100,50]  打印-1
        let arr1 = [88, 20, 10, 100, 50]
        let arr2 = [88, 20, 30, 100, 50]
        for (i = 0; i <= arr1.length; i++) {
            if (arr1[i] === 10) {
                document.write(`第${i+1}个元素是10`);
            } else {
                document.write(`-2`);
            }
        }
        for (i = 0; i <= arr1.length; i++) {
            if (arr2[i] === 10) {
                document.write(`第${i+1}个元素是10`);
            } else {
                document.write(`-2`);
            }
        }
    </script>
</body>

</html>